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The Extralight Fusion Engine (often shortened to XL Fusion Engine) is mechanically similar to a Fusion Engine but uses much lighter radiation shielding. Replacing the dense tungsten carbide of standard fusion engines with a crystalline polymer similar to that of double heat sinks, the XL fusion engine is half the mass for the same performance.

There is a penalty in bulk, however. The Inner Sphere XL fusion engine is twice as large as the standard fusion engine, while the Clan model is about two-thirds larger. The larger volume of the engines makes them much more likely to end up in the path of penetrating hits. Further, these bulkier engines intrude into the side torso bays of a BattleMech, exposing the 'Mechs to catastrophic engine damage from structural damage that would be otherwise survivable.

There is also a penalty in cost; an XL fusion engine is four times as expensive as a standard, a component that is already usually the largest cost in a BattleMech. Despite this, the allure of huge weight reduction means that the technology sees wide use (as evidenced by the long list of manufacturers below).

Developed by Terran Hegemony scientists, these power plants first saw use by their armed forces in 2579. By the time Kerensky left in 2784, they were already widely deployed; the subsequent chaos of the Succession Wars caused the destruction of the last XL engine manufacturing plants by 2865. The technology wouldn't resurface in the Inner Sphere until 3035, when it was rediscovered by the Lyran Commonwealth. [1]

It has been indicated that the Clan model of Extralight Fusion Engines requires special materials only available from facilities on the Clan homeworlds. Wolf's Dragoons spent several years building their own engines for their OmniMechs before this material shortage compelled them to find an alternative. Rather than using the bulkier (and more vulnerable) Inner Sphere engines, they successfully developed the Light Fusion Engine, which uses the same bulk of the Clan models but at half the weight savings. This is notable as it seemed the Dragoons had little trouble reproducing other Clan equipment. [2]